This invention relates generally to pressurized fluid-containing devices such as used in the inflation of inflatable devices such as inflatable vehicle occupant restraint airbag cushions and, more particularly, to the introduction of a leak trace material, particularly a radioactive leak trace material, into such pressurized fluid-containing devices.
It is well known to protect a vehicle occupant using a cushion or bag, e.g., an "airbag cushion," that is inflated or expanded with gas when the vehicle encounters sudden deceleration, such as in the event of a collision. In such systems, the airbag cushion is normally housed in an uninflated and folded condition to minimize space requirements. Upon actuation of the system, the cushion begins to be inflated in a matter of no more than a few milliseconds with gas produced or supplied by a device commonly referred to as an "inflator."
The above-referenced prior U.S. patent application Ser. No. 08/935,016 relates that various inflator devices have been disclosed in the art and that it is common for various such inflator devices, or at least particular components thereof, to be checked for the presence or occurrence of undesired leaks. In particular, one category of inflator devices is often referred to as "compressed gas inflators" and refers to various inflators which contain compressed gas. One type of compressed gas inflator, commonly referred to as a "stored gas inflator," simply contains a quantity of a stored compressed gas which is selectively released to inflate an associated airbag cushion. In a second type of compressed gas inflator, commonly referred to as a hybrid inflator, inflation gas results from a combination of stored compressed gas and the combustion of a gas generating material, e.g., a pyrotechnic.
Compressed gas inflators commonly require the presence of at least certain specified quantities of the particular compressed gas material in order for the inflator to perform in the designed for manner. In such inflators, it is generally desired that the amount(s) of stored compressed material(s) be maintained in the inflator within at least a certain specified tolerance in order to ensure proper operation of the inflator. While proper inflator operation can be variously defined, ultimately an inflator and associated airbag cushion need provide adequate vehicle occupant protection over an extended period of time (typically 15 years or more) subsequent to original construction and installation in a particular vehicle. Thus, beyond the simple functioning of the inflator and deployment of an associated airbag cushion, such inflatable restraint systems desirably operate or function in a manner wherein the airbag cushion will deploy, when needed, in the desired and proper manner.
While there are various methods to determine the rate of leakage from a compressed gas inflator, in practice, a typically preferred method relies on the incorporation of helium as a tracer gas in the particular compressed gas mixture. In such a method, helium will constitute a certain fraction of the stored gas composition which escapes from the inflator. (As will be appreciated and dependent on the specific situation, the exact fraction of helium detected as a result of a leak may be equal, less than, or greater than the helium fraction of the stored compressed gas. The physics associated with these various situations, however, is generally beyond the scope of the present discussion. Typically, however, these different situations are dependent on certain, particular factors such as the magnitude of the leak, the total pressure within the storage vessel, as well as the initial gas composition, for example.)
The rate of helium leakage from a pressure vessel is normally detected using a mass spectrometer system. For this specific practice, the mass spectrometer is normally calibrated or designed to detect the presence of helium in the gases constituting the sample. The utilization of helium as a leak trace material is advantageous in several respects:
First, as the normal or typical atmospheric content of helium is rather low, the background helium level (or residual helium in the environment such as that surrounding the detection apparatus) is normally correspondingly low. As a result, the possibility of a corresponding mass spectrometer being falsely influenced and possibly producing a spurious signal is significantly reduced or minimized.
Second, the signals of a mass spectrometer for at least certain different molecular species can be nearly the same. Consequently, a mass spectrometry signal produced or resulting from the presence or occurrence of one molecular species may interfere or mask a mass spectrometer signal produced or resulting from the presence or occurrence of a different molecular species. For example, the molecular weights of nitrous oxide and carbon dioxide are approximately 44.02 and 44.01, respectively. As a result, it is generally very difficult to distinguish between these molecular species via mass spectrometry. Helium, however, with a molecular weight of 4, produces a mass spectrometry signal that is relatively easily distinguishable from the signal correspondingly produced by other potentially present species.
Third, helium is a relatively small, low molecular weight monatomic gas, facilitating the passage thereof through even relatively small or narrow leak paths. Thus, such use of helium may facilitate or better permit detection of even relatively small or narrow leak paths.
Conventional helium leak detection techniques may, however, suffer or potentially suffer from a number of problems or possible disadvantages. For example, in order to permit a leak check or determination of the relatively small range of leakage which may normally be acceptable for commercial airbag inflator devices, it is commonly necessary to include a relatively large amount of helium in the associated compressed gas mixture. In practice, the amount of helium required is generally dependent on factors such as the magnitude and type of leak, the design life of the inflator, and the criteria for adequate performance for the inflator as a function of time. However, the incorporation of even moderate amounts of helium within a compressed gas inflator is or can be disadvantageous as such inclusion can, for a given volume, significantly increase the storage pressure of the corresponding inflator contents. Conversely, at a given pressure, the storage volume of an inflator will need to be increased in order to accommodate the mass of the so added helium.
A significant limitation on the use of helium in such leak detection schemes is that unless the helium concentration within the vessel is known, the leak rate from an inflator pressure vessel normally cannot be accurately checked at a date substantially later than the date the inflator was manufactured. That is, unless the leak is of the type that the compressed gases (e.g., both the primary stored gas and the helium tracer gas) are escaping in equal proportion to that at which they were loaded (as in the original composition), then the leak rate determination will normally be in error. Since knowledge of the type of leak cannot be definitively known a priori, the making of such an assumption can result in significant error. Moreover, if a pressurized vessel is returned at a later date for the leak rate to be reevaluated, a helium leak rate determination may be inaccurate.
An additional possible limitation or drawback to the use of such helium leak detection techniques is that the occurrence or presence of liquid materials within the storage vessel may impede or "mask" the helium. For example, if a liquid with a relatively high surface tension is present in the vessel, such liquid could possibly flow into a hole through which gas would normally leak and may, at least temporarily, inhibit the passage of the gaseous material out of the inflator. However, with time, the liquid may no longer occupy the leak path and the stoppage of gas leakage therethrough may only be temporary.
In addition, though helium is relatively rare in the general atmosphere, it will be appreciated that relatively high background concentrations of helium can be created in manufacturing environments. This may necessitate that a vessel being tested be isolated such as by being placed in a closed chamber in which a vacuum is created in the surrounding environment, with the helium leak rate then being determined. Such special handling can significantly increase the time and expense associated with the manufacturing process.
Further, the use of and reliance on such helium leak detection techniques may undesirably result in the addition of considerable expense to the cost of the inflator, both through the inherent cost of helium and the cost of purchasing and maintaining the mass spectrometers, as well as the costs associated with the equipment required to store, mix, and handle the helium.
Thus, there is a need and a demand for a pressurized fluid-containing inflator design which facilitates leak detection. To that end, the above-referenced prior U.S. patent application Ser. No. 08/935,016 discloses the development of a method for the detection of the occurrence of a leak from an otherwise closed chamber which contains a pressurized fluid. In accordance with one such method, a selected quantity of at least one radioactive isotope leak trace material is included within the particular inflator chamber. The detection of the occurrence of a leak from the chamber is done by measuring the reduction or change in the radioactive signals emanating from the chamber.
As disclosed therein, such methods may provide a convenient and accurate means by which to leak check associated or corresponding pressurized, fluid-containing chambers or vessels, such as may be included in an airbag inflator. Further, such methods may correct problems and alleviate disadvantages, both actual and potential, which are inherent in the common application of helium as a leak detection material; including, for example, the increased size, weight and cost of such inflators as well as the increased manufacturing, equipment and personnel costs and expenses such as may be associated with the application of helium as a leak detection material. Still further, such invention may provide for the avoidance or minimization of possible leak paths from such pressure vessels, particularly pressurized fluid-containing airbag inflators. Yet still further, such invention may provide an apparatus and method wherein a material used to accomplish a leak check can, if desired, be directly included with a fuel source, such as to aid or assist in dissociation. Yet still even further, such invention may provide alternative and possibly safer, simpler or less costly techniques by which inflator devices can be appropriately filled with desired gas and liquid fluid materials.
Certain limitations or restrictions, however, have or may present obstacles to the greater or more widespread use of and reliance on leak detection and measurement via such radioactive materials. For example, in order to improve either or both the efficiency and accuracy of leak detection and measurement via such radioactive materials, there is a need and a demand for methods or techniques for better ensuring that such leak trace materials and pressure vessel contents form or are otherwise present as a substantially homogeneous mass within a respective pressure vessel chamber which is to be leak checked by the measurement or detection of radioactive signals emanating from such a chamber. In particular, there is a need and a desire for a simple method of preparing or loading such leak trace materials within such pressurized chambers and such as may result or better ensure that the leak trace material and other pressure chamber contents form or are present as such a homogeneous mass.
Further, as disclosed in the above-referenced prior U.S. patent application Ser. No. 08/935,016 a quantity of such radioactive leak trace material can be held by a solid material within a particular pressurized, fluid-containing chamber or vessel of an inflator device. Such solid material may desirably initially hold at least a substantial portion of the radioactive isotope leak trace material contained within the particular chamber or vessel. As disclosed, a large percentage of the leak trace material originally absorbed or held by or in the solid material will over time be gradually released or no longer held thereby and thus permit or allow the detection of the presence of leaks, in accordance with the corresponding leak detection process.
Such a solid material which initially holds at least a substantial portion of the radioactive isotope leak trace material can be of various compositions. For example, many decomposition sensitizer materials can also advantageously serve as such a radioactive isotope leak trace material-holding solid, also commonly referred to as a "getter."
A potential complication or shortcoming relating to the employment of a radioactive isotope leak trace material-holding solid may arise from the period of time required for the leak trace material to release and subsequently pass or diffuse from the "holding" solid into the stored fluid of the particular chamber or vessel. As will be appreciated, should the holding solid not release or allow the leak trace material to pass or diffuse into the stored fluid of the particular chamber or vessel in the proper manner, an erroneous leak detection measurement may result.
Thus, a need and a demand exists for improved methods for introducing a radioactive leak trace material into an apparatus for inflating an inflatable device and such as may reduce, minimize or overcome the potential for erroneous leak detection measurement such as may be associated with a holding solid not releasing or allowing a leak trace material to pass or diffuse into the stored fluid of the particular chamber or vessel in the proper manner.